Insulators or dielectrics have widespread use in semiconductor processing. Silicon dioxide films are best known for their use as passivation to provide physical and chemical protection to the underlying devices and components. Silicon nitride films have also been widely used as a passivation layer providing additional scratch protection due in part to its hardness. The role of silicon dioxide films in processing has expanded. Deposited silicon dioxide films are now used as interlevel dielectrics between polysilicon and metal lines, as isolation films, as dopant barriers and as diffusion sources.
Deposited silicon dioxide films have a different physical structure from thermally grown oxide films. Depending on the deposition temperature, the silicon dioxide may have, among others, a different density, dielectric strength and etch rate. The addition of dopants to deposited silicon dioxide films may change the chemical and physical properties of the films. Deposited silicon dioxide films may also undergo a process called densification. The deposition of films was started to allow for a low-temperature deposition to occur preventing undesirable redistribution of impurities in the underlying regions during the processing steps. The densification of the silicon dioxide after deposition forms a film with physical and chemical properties approximating that of thermally grown oxide films.
There are several benefits of adding dopants to the silicon dioxide films. The moisture barrier properties of the films increase. Contaminants are prevented from entering the underlying layers and the viscosity of the films increase. This last benefit of increasing the flow property enhances the planarization of the surface of the film. Typically, boron and phosphorous are added to the silicon dioxide to enhance the flow property. The resultant film is known as borophosphorous silicate glass (BPSG).
As the concentration of dopants increases in the glass film, the temperature at which the film will reflow decreases. The lower processing temperature to cause reflow will not effect the electrical performance of the devices and components. As additional dopants are added, however, the surface of the glass layer becomes dopant rich. This increased concentration at the surface causes adhesion problems during subsequent contact patterning processes. In other words, the ability of photoresist to adequately adhere to the doped glass layer is significantly reduced. After the photoresist is formed over the glass layer and patterned, the opening formed in the photoresist is cleaned to remove any remaining photoresist residue in the areas where a contact is to be etched. This process, called descuming, enhances the photoresist's ability to adhere to the underlying glass layer. However, during the process of cleaning, a portion of the sidewalls of the photoresist is also removed. The removal of any of the photoresist along the sidewalls is becoming unacceptable in the submicron geometries.
It would be desirable to provide a technique which increases the adhesion of photoresist to the underlying dielectric layer. It would further be desirable for such a technique to be easily adapted for use with standard integrated circuit fabrication process flows without increasing the complexity of the process.